[6.x] Fix Model::withoutEvents() not registering listeners inside boot()

[derekmd]

Fixes #32935

Eloquent model boot() hooks (and booting()/booted() in 7.x+) run once per-class in each request. If an Eloquent model class is instantiated for the first time inside Model::withoutEvents(), boot() callbacks registering custom event listeners expected later will be skipped. e.g.,

class Invoice extends Model
{
    public static function boot()
    {
        parent::boot();

        static::creating(function (Invoice $invoice) {
            if (blank($invoice->number)) {
                $invoice->generateNumber();
            }
        });
    }
}

public function test_it_will_auto_generate_unique_invoice_number()
{
    $firstInvoice = User::withoutEvents(function () {
        // Wish to avoid firing listeners from User model events.
        $user = factory(User::class)->create();

        return Invoice::create([
            'user_id' => $user->id,
            'number' => 'I'.Str::random(5),
        ]);
    });

    // This doesn't fire the 'creating' listener to auto-fill database column
    // "invoices"."number". `Invoice` above had no event dispatcher so the
    // 'creating' listener was never registered.
    $secondInvoice = Invoice::create(['user_id' => $firstInvoice->user_id]);

Instead of Model::withoutEvents() removing the event dispatcher from all models, replace it with a null pattern implementation. Fired event dispatch() attempts are noop when inside withoutEvents(). However event listeners can still be registered on the concrete dispatcher for post-withoutEvents() calls.

[derekmd]

Packagist and/or GitHub hosting seem to be eating it at the moment so CI step "Install dependencies" for Composer is failing from network timeouts.

[taylorotwell]

Thanks